Abstract

We analyze dark current-voltage and short-circuit current versus open-circuit voltage measurements of hydrogenated amorphous siliconpin and nip diodes. The ideality factor of ∼1.4 is independent of the thickness of the intrinsic layer, indicating that recombination occurs in the interface region rather than in the bulk. These results can be simulated accurately when the defect-pool model is used. Lateral amorphous silicon diodes with a uniform defect density throughout the intrinsic region have ideality factors, which do depend on the size of the intrinsic region and exceed the theoretical value of 2. We present a model to explain the ideality factor and to verify the results with numerical simulations. Values smaller than 2 are caused by unequal shifts of the quasi-Fermi levels for electrons and holes; values larger than 2 arise when the recombination is spread out over a wide region. In that case the distance between the quasi-Fermi levels is smaller than the applied voltage.